The Committee on Nutrition of the American Academy of Pediatrics suggests that a logical goal for nutritional support of the LBW infant is to achieve postnatal growth approximating that of a normal fetus of the same post-conceptional age. To date, attempts to achieve sufficient catch-up growth in the LBW infant to result, at discharge, in the infant's weighing the same and having the same body composition as a fetus of comparable post-conceptional age have failed. Our previous studies suggest that this is because the relatively high protein intake required is not utilized completely unless accompanied by an energy intake that results in excessive fat deposition. We, therefore, propose to evaluate a strategy for enhancing accretion of lean body mass without excessive fat accretion. Specifically, we will test the hypothesis that a high-carbohydrate diet relative to an isocaloric high-fat diet will enhance protein utilization but may exert undesirable effects on cardiorespiratory function. To test this general hypothesis, we will examine systematically the differential effects of high-carbohydrate and high-fat energy intakes on weight gain, nitrogen retention, ratio of tryptophan to concentrations of large neutral amino acids, sleep state, substrate utilization, energy expenditure and cardiorespiratory functions of enterally fed, growing LBW infants (Birth weight, 750-1600 g). All infants will receive a protein intake of 4.0 g/kg.d; concomitant energy intake will be either 120 kcal/kg.d (non-protein energy 65% carbohydrate and 35% fat, 35% carbohydrate and 65% fat or 50% carbohydrate and 50% fat) or 150 kcal/kg.d (non-protein energy 65% carbohydrate and 35% fat or 35% carbohydrate and 65% fat). The question whether fat and carbohydrate are equivalent sources of energy is not only important for the nutritional management of LBW infants but may also be relevant to their neurobehavioral development.